In the tire for heavy vehicles, e.g. truck, bus and the like, a square shoulder in which, as shown in FIG. 4, an outwardly swelling tread face (a) and inwardly swelling buttress faces (b) intersect at the tread edges at about a right angle, has been used.
Such a square shoulder has a merit of an even ground pressure distribution, and thereby uneven wear becomes less, and the wear life of the tread is excellent.
In tires having the square shoulder, however, the tread edges are liable to be damaged during running on rough roads, and further the directional stability is not good. That is, wandering of the vehicle occurs during running on a road having a surface irregularly inclined in the lateral direction, for example, a rutted road.
The countermeasures for such problems, which have been widely employed, are to use:
a round shoulder in which the tread edge portion (c) is rounded with an arc (R) as shown in FIG. 5; and
a taper shoulder in which the tread edge portion is formed by a tapered face (e) as shown in FIG. 6.
However, in tires having the round shoulder, as the ground pressure is decreased in a region near the tread edge, shoulder wear, in which the tread shoulder portions are worn rapidly in comparison with the tread crown portion, occurs. On the other hand, in the tires having the taper shoulder, the improvement in the wandering performance is not sufficient.